index: fix confusion with shared prefix in compressed path names

The index version 4 introduced compressed path names for the entries.
From the git.git index-format documentation:

    At the beginning of an entry, an integer N in the variable width
    encoding [...] is stored, followed by a NUL-terminated string S.
    Removing N bytes from the end of the path name for the previous
    entry, and replacing it with the string S yields the path name for
    this entry.

But instead of stripping N bytes from the previous path's string and
using the remaining prefix, we were instead simply concatenating the
previous path with the current entry path, which is obviously wrong.

Fix the issue by correctly copying the first N bytes of the previous
entry only and concatenating the result with our current entry's path.

src/index.c

@@ -2355,21 +2355,24 @@ static size_t read_entry(
 		entry.path = (char *)path_ptr;
 	} else {
 		size_t varint_len;
-		size_t shared = git_decode_varint((const unsigned char *)path_ptr, 
-						  &varint_len);
-		size_t len = strlen(path_ptr + varint_len);
+		size_t strip_len = git_decode_varint((const unsigned char *)path_ptr,
+						     &varint_len);
 		size_t last_len = strlen(*last);
-		size_t tmp_path_len;
+		size_t prefix_len = last_len - strip_len;
+		size_t suffix_len = strlen(path_ptr + varint_len);
+		size_t path_len;
 
 		if (varint_len == 0)
 			return index_error_invalid("incorrect prefix length");
 
-		GITERR_CHECK_ALLOC_ADD(&tmp_path_len, shared, len + 1);
-		tmp_path = git__malloc(tmp_path_len);
+		GITERR_CHECK_ALLOC_ADD(&path_len, prefix_len, suffix_len);
+		GITERR_CHECK_ALLOC_ADD(&path_len, path_len, 1);
+		tmp_path = git__malloc(path_len);
 		GITERR_CHECK_ALLOC(tmp_path);
-		memcpy(tmp_path, last, last_len);
-		memcpy(tmp_path + last_len, path_ptr + varint_len, len);
-		entry_size = long_entry_size(shared + len);
+
+		memcpy(tmp_path, last, prefix_len);
+		memcpy(tmp_path + prefix_len, path_ptr + varint_len, suffix_len + 1);
+		entry_size = long_entry_size(prefix_len + suffix_len);
 		entry.path = tmp_path;
 	}